Engine size effects on cam spec

[spoolin70]

Sunday morning musings - how does an engines size effect the cam spec ?

Let's say cam X is designed for 1776 - 1915 to give around 150bhp.

If the same cam is then fitted to a 2276, what happens to the speculated bhp figure ?

If displacement alone is increased, will the bhp stay the same and be reached at a lower rpm ? Or is it presumed that an increase in displacement will include larger valves, higher rocker ratio etc so the 150bhp could increase to 180bhp ?

Sitting in the sun, drinking coffee, thinking about this
Darren

[nicolas]

in short: the bigger the engine the 'tamer' the cam becomes. it will work at lower RPMs


this can be a great sunday post indeed , too bad we have rain here. 

[spoolin70]

Thanks for the reply Nicolas

Ok so can I add another element to consider. If cam X was fitted to a turbo/injection engine of a larger capacity, could the tamer characteristics be reduced/eliminated by adding boost and fuel ?

Sun is out again 
Darren

[nicolas]

well here we have buckets of rain (again)

i know that wilder cams can be made more 'docile' with EFI. but that is because more parameters can be controlled to tune the fuel and trottle opening, vacuum signal,… more refined as opposed to carbs with 3-4 different circuits.
as for boost, i know there are quite a few 'very tame cammed' turbo engines out there (like a W100) but making tons of power. why? i don't know either, but you have opened the debate and i am sure this blank will be filled in. no matter if it is sunny or rainy 

[Fiatdude]

I had a web 86c with 1.5 rockers in my 2989, and it acted like a stocker... and when I added boost Yee Hah -- --

A buddy and I were having this exact same conversation last week and just couldn't figure out why stock cams seem to work the best under boost......

[Jim Ratto]

Sunday morning musings - how does an engines size effect the cam spec ?

Let's say cam X is designed for 1776 - 1915 to give around 150bhp.

If the same cam is then fitted to a 2276, what happens to the speculated bhp figure ?

If displacement alone is increased, will the bhp stay the same and be reached at a lower rpm ? Or is it presumed that an increase in displacement will include larger valves, higher rocker ratio etc so the 150bhp could increase to 180bhp ?

Sitting in the sun, drinking coffee, thinking about this
Darren

I'm glad somebody posted this question and I wished the people writing the cam catalogs paid more attention to this. I've complained for a long time about how misleading the descriptions are in cam catalogs, which really can't be called "misinformation" but it could well be referred to as "incomplete information." 

You can bolt any cam into any engine, meaning if you order a Type 1 cam, you can install it in any (size cc) Type 1 engine, provided gear/cam and oil pump all are made to get along. Problem is, the cam catalog people tend to write the descriptions just like this.

Cam "X" could be 0.470" at valve, 257 deg @ .050" and 288 deg advertised, I'm just making this up for an example on what we could all agree on as a hot street-ish cam.
We should probably get some of the requirements discussed before we theorize how it's going to behave...

1. dual port heads MINIMUM
2. dual 2bbl carbs or some kind of FI and an exhaust that creates strong vacuum signal
3. some dynamic compression (how much depends on cam timing, cc of engine, intended use, port work, intake, etc)
4. Robust enough valvetrain so everything stays next to one another and so it can do some revs

If you were to build an 87 x 69 with the above cam, set up with correct port design/diameter, valve diameter, dual 36-40mm IDF/DRLA, 9.5:1 and some kind of appropriate extractor header, you can be sure it's going to want to run, and run hard in the upper rpm reaches. Because of the IR intake, it will actually run well all around, but definitely come into its own after 35-3800 rpm. I ran this engine years ago, as a daily driver. I'd have to keep the gearshift moving, to keep the thing in the sweet spot, but it was a lot of fun.

If you used this cam in a big 2300cc romper stomper, the character of the engine would change dramatically. The requirements would remain the same, but the engine would now probably "hit hard" down around 2300-2700 rpm, and pull well to 58-6000 rpm. Not as much need to shift down, but maybe less fun to wring out on a long open road.


As cylinder cc volume goes up, in order to fill them, you need more cam to keep higher rpm VE up.

As Porsche increased 911 displacement, but retained same "S" cam profile, peak HP kept happening at a lower and lower rpm. And their specific hp went down. I'm not saying the engine HP went down, but the engine's hp/liter did.

[modnrod]

I once built an Aussie Holden 253 V8 using a popular (coz it worked) Crane CC276. It idled OK, and with pocket port stock heads/maifold/4BBL/tri-Ys it pulled hard to 6000 easy. A really lovely little street motor.
When I changed it to a 308 V8, I also upgraded the heads (and therefore the flow........), intake manifold, etc. I did all this expecting what you have just said Jim, about needing a bigger cam to compensate for the extra displacement, but I found to my surprise that the 308 actually pulled HARDER and higher using the same cam. I had raised the airflow speed and volume much higher than the 20% capacity boost of the little motor.

I think these old VWs need more airflow for the extra capacity obviously, but in this case the easiest way to do it is to add area under the curve rather than extra speed and port flow. Really strong well done heads are expensive, an extra 10* duration and another compression point are cheap.

[spoolin70]

Thanks for the info guys - I like the real world numbers 

Here's some engine and cam info -

86 x 101.6 (2789cc)
JPM MS250 heads - 48 / 38 valves. Flow at 25" H2O: In 255cfm   Ex 180cfm
                                                          Flow at 28" H2O: In 270cfm   Ex 190cfm
Berg 1.4 rockers.
8.5 : 1 static compression.
Turbo + injection.

Cam - lift at 0 lash - 9.57mm intake & exhaust.
Lobe angle - 56.5 degree intake & exhaust.
Lobe centre 113 degree
Duration at 1.27mm - 258 intake & exhaust.
Duration at lobe lift - 1.27mm
Timing - (open) in 16 / ex 62

This was apparently spec'd for 600bhp but in a 2332 engine. I'm putting it in my 2789 and it's the increase in displacement that's got me wondering. 

Thanks again
Darren

[Stripped66]

This was apparently spec'd for 600bhp but in a 2332 engine.

Can you consult the guru who spec'd that cam for that engine? Did you get the turbo that went with that combo, too?

[spoolin70]

I have asked the person who cut the cam and the response was "Normally a bigger engine needs a bigger cam, but maybe you can increase the boost and get the same numbers".

I'm sure you can achieve a number by just increasing boost but it was the characteristics of the cam and how it performs the rest of the time that got me wondering - this is a street car at the end of the day.

I don't have the turbo but I have most of the numbers for it. I know someone that can make turbos to spec so hoping that isn't a problem.

What are you thinking about the cam spec then Stripped66 ?

Cheers
Darren

[fish]

Your cam looks good to keep some off boost torque for that big lump, great for the street.
When in doubt and not getting clear answers from the cam grinder, talk to Dave Kawel or Don Pauter.

[spoolin70]

Good shout Fish

I've just spent some money with Pauter so I reckon I've earnt some technical advice 

Just sent some info over to Mario at Pauter - will post the verdict

Darren

[spoolin70]

Word back from Mario -

Street & strip car cam choice is ok.
Bigger engine can tolerate more duration for better early power and keep RPM to minimum and create longevity ..

A bit less detail than I had hoped for but still positive.

Specs have been sent to Piper cams also for another view on things 

[spanners]

A bit off topic, but certainly cam performance related, What hot valve lash are you air cooled turbo guys seeing? Both with cm and ally push rods? I get considerable heat sink AFTER racing, often, then still red hot, I have to go and perform again, my cm pushrods leave me with .015" hot lash during this phase, too much lift loss = performance ditto, I feel a swap back to alloy coming on to address this.

[Roman]

There is more than duration and lift on a cam. You also have to consider the jerk - more or less how steep the lift curve is.
Talk to JPM, he made the heads and he also makes custom camshafts.

[cassa]

I would contact Tor Øyvind Skinne if you need a correct camshaft/turbo for your engine. He has many cams on the shelf that will suit your engine. PM me for contact information.
Reg Carsten

[spoolin70]

Hi Roman
The cam is from JPM and it was Johannes I spoke to first. He was the person who said I could 'maybe' get the same numbers from turning the boost up.

I own a few of his parts so I believe in the products but I had hoped for a more accurate answer or is there are so many variables is it just that hard to predict how a cam will behave and what results it will give ??

Thanks
Darren